During electrolysis, for example, electric power is converted into chemical energy. This is achieved through the decomposition of a chemical compound by means of an electric current. The solution used as electrolyte contains positively and negatively charged ions. Therefore, mainly acids, bases or salt solutions are used as electrolyte.
For example, in the case of the electrolytic production of halogen gases from aqueous alkali halide solution—here represented by sodium chloride—the following reaction takes place on the anode side:4NaCl→2Cl24Na++4e−  (1)The liberated alkali ions move to the cathode where they form caustic soda with the hydroxide ions generated there. Hydrogen is also formed:4H2O+4e−→2H2+4OH−  (2)The caustic soda produced is separated from the alkali halide fed to the anode side by means of a cation exchange membrane, separation thus being achieved. Cation exchange membranes are state-of-the-art and commercially available from a number of different suppliers.
The standard potential generated at the anode by the formation of chlorine when reaction (1) above takes place is +1.36 V, with the standard potential at the cathode being −0.86 V when the above reaction takes place. A cell design of this type is known, for example, from WO98/55670. The difference between these two standard potentials yields an enormous influx of energy, which is required for these reactions to take place. In order to minimise this difference, gas diffusion electrodes (hereinafter referred to as GDEs) are used on the cathode side, meaning that oxygen is fed into the system with the result that reaction (2) is replaced by the following reaction at the cathode:O2+2H2O+4e−→4OH−  (3)Oxygen can be supplied as pure gas or as air. The following overall reaction for chlor-alkali electrolysis using gas diffusion electrodes results:4NaCl+O2+2H2O→4NaOH+2Cl2  (4)As the standard potential of reaction (3) is +0.4 V, GDE technology results in a considerable power saving compared with conventional electrolysis with hydrogen formation.
Gas diffusion electrodes have been used for many years in batteries, electrolysers and fuel cells. The electrochemical conversion takes place inside these electrodes at the so-called three-phase boundary only. Three-phase boundary is the term used for the area where gas, electrolyte and metallic conductor meet. For the GDE to work effectively, the metallic conductor should also be a catalyst for the desired reaction. Typical catalysts in alkaline systems are silver, nickel, manganese dioxide, carbon and platinum. To be particularly effective these catalysts must have a large surface area. This is achieved by finely divided or porous powders with an inner surface.
Problems in the use of such gas-diffusion electrodes as disclosed in U.S. Pat. No. 4,614,575, for example, are due to the fact that the electrolyte would penetrate into these fine pore structures due to capillary action and fill them up. This effect would make the oxygen stop diffusing through the pores, thus stopping the intended reaction.
For the reaction to take place effectively at the three-phase boundary, the above problem must be avoided by selecting the pressure ratios accordingly. A static liquid column would otherwise be formed in the electrolyte solution, which would cause the hydrostatic pressure to be highest at the lower end of the column, thus enhancing the phenomenon described above.
As documented in the relevant literature, this problem is solved by using falling-film evaporators. Here, the electrolyte, such as caustic soda solution NaOH or caustic potash solution KOH, is percolated through a porous material positioned between the membrane and the GDE, thus preventing the formation of a hydrostatic column. This is also referred to as percolation technology.
WO 03/42430 describes such an electrolysis cell, which utilises this principle for the chlor-alkali electrolysis reaction with an oxygen consumption reaction. Here, the oxygen is separated from the porous material by the gas diffusion electrode, and the GDE and the porous material—the percolator—are pressed together by means of a conductive support structure and a conductive flexible spring element.
The same kind of principle can also be found in DE102004018748, for example. This describes an electrochemical cell consisting of at least one anode compartment with an anode, a cathode compartment with a cathode and an ion exchange membrane arranged between the anode compartment and the cathode compartment, the anode and/or cathode being a gas diffusion electrode, and there being a gap between the gas diffusion electrode and the ion exchange membrane, an electrolyte inlet above the gap and an electrolyte outlet below the gap as well as a gas inlet and a gas outlet, the electrolyte inlet being connected to an electrolyte feed tank and having an overflow.
However, use of the gas diffusion electrode in the electrolytic apparatus described is not merely for the purpose of allowing the catalytic oxygen consumption reaction. The aim of the electrode is also to ensure the separation of electrolytes and gas on both sides of the GDE. For this, it is imperative to seal the gas diffusion electrodes gas tight and liquid tight by means of the selected anchoring method in order to ensure—particularly once the electrolyte has entered the cell—that the electrolyte is routed along the gas diffusion electrode as intended and does not reach the electrolyte outlet from the electrochemical cell via untight areas and thus alternative routes, consequently not being available for the reaction.
As the gas diffusion electrodes are subject to ageing and thus to wear, they must be replaced after a given operating period. The prior art envisages the gas diffusion electrodes being welded into the cathode compartments, which makes them work-intensive to replace.
This is elaborated, for example, in DE 103 30 232 A1. This describes an electrochemical compartment in which the GDE has an uncoated periphery that is connected to an anchoring structure equipped with an electrically conductive plate. This method of anchoring a GDE, which at the same time allows the electrolyte space to be sealed off from the gas space is, if anything, disadvantageous in combination with a percolator as it may lead to the percolator material being damaged and the electrolyte flow across the percolator being blocked. Moreover, when assembling this type of arrangement it is essential that the GDE is pushed an exact even amount under the electrically conductive plate 3 across the entire width of the electrochemical cell as otherwise the electric plate unevenly changes the free cross-sectional area available for liquid flow in the percolator arranged parallel thereto, which means that there is no guarantee that the liquid will be distributed evenly, which is essential if the electrochemical cell is to work properly. Ensuring even distribution is very difficult with an arrangement of this type.
An alternative method of anchoring the gas diffusion electrodes is defined in DE 101 52 792. This document describes a method for connecting a gas diffusion electrode to the basic structure of the electrolytic apparatus by means of an enclosing bent frame. As a pure clamping method, this method is more advantageous with regard to replaceability than that described in DE 103 30 232. However, as the frame and the basic structure are in this case also connected by welding or soldering in order to minimise ohmic losses, there are still the disadvantage regarding difficult replaceability and the loss of active electrode surface area due to welds.
It is therefore the objective of the present invention to find an alternative method for anchoring the gas diffusion electrode in an electrochemical cell to ensure ease of installation and removal, adequate sealing off of the gas space from the electrolyte space and provision of the biggest possible active electrode surface for the electrochemical reaction.